This invention relates to a semiconductor diode.
A semiconductor diode is known which has a first region of one conductivity type, a second region of the opposite conductivity type meeting only a given surface of the body and surrounded by the first region so as to form with the first region a first pn junction which, when reverse-biassed in operation of the diode by a voltage applied across the diode, gives the diode a blocking characteristic and a third region of the one conductivity type more highly doped than the first region provided within the first region for triggering conduction of the device when a predetermined voltage less than the voltage at which the main pn junction would have broken down in the absence of the third region is applied across the diode to reverse bias the first pn junction.
Such a semiconductor diode is described in, for example, GB-B-2113907. In particular, GB-B-2113907 describes a four layer pnpn semiconductor diode having a gateless thyristor structure. The gateless thyristor structure has a p type anode region, an n type central region, a p type central region and an n type cathode region. The two or first and second central regions form the first pn junction which is reverse-biassed when a forward voltage is applied across terminals connected to the anode and cathode regions. When such a forward voltage is applied across the gateless thyristor structure the structure maintains a blocking characteristic until the thyristor structure is triggered into conduction by breakdown of the reverse-biassed first pn junction or by a large capacitive current flowing in the central p region under the cathode region because of a rapid rise in the potential across the electrical contacts. Accordingly, such diodes can be used, for example, to protect electrical equipment from supply voltage spikes that may be dangerous because of the high voltage or because of the rapid rise in voltage.
GB-B-2113907 also describes a gateless triac structure consisting of two anti parallel gateless thyristors. Such a structure may also be known as a symmetric breakover diode. Assymmetric breakover diodes which have a thyristor structure in one direction and a two-layer diode structure in the opposite direction are also available. The I-V characteristic of an assymmetric breakover diode is shown in FIG. 1a of the accompanying drawings. The I-V characteristic of a symmetric breakover diode differs from that of an assymmetric breakover diode in that the characteristic is the same in the first and third quadrants in the case of the symmetric breakover diode.
In order for a semiconductor diode having the gateless thyristor structure described above to provide protection against a rapid voltage rise caused by, for example, a main supply voltage spike, a suitable choice of sheet resistance under the cathode region needs to be selected by determining the depth of the central or inner p and the cathode regions and the surface dopant concentrations of those regions. Previously, in order to provide protection against high voltage, that is to cause the first pn junction to breakdown to trigger conduction in the device when a given voltage is applied across the electrical contacts, the doping level of the central n region or substrate has been adjusted. However, adjustment of the doping of the central n- region is not particularly desirable because it means that various substrate for forming the n-type central region are required which have different doping concentrations to enable devices providing protection against different high voltages to be produce. As recognized in GB-B-2113907 it is not desirable to have to rely on the reverse-biassed first pn junction to control the voltage at which the semiconductor diode breaks down because breakdown of the first pn junction occurs at a relatively unpredictable voltage where the junction meets a surface of the p and n regions and therefore non-uniformities in triggering of the diode may occur. A further problem not apparently recognized in GB-B-2113907 is that, in the case of an assymetric breakover diode in particular, as the doping level of the central n- region or substrate is increased a point is reached when the transverse component of the breakdown current no longer produces a voltage sufficiently high to turn on the bottom pn junction of the thyristor structure so that true thyristor action does not occur.
Thus, as will be appreciated from the above, controlling of the breakdown of a reverse-biassed first pn junction in a semiconductor device such as a breakover diode merely by controlling the doping of one region of the first pn junction, for example the substrate in the example given above, is undesirable and becomes impracticable if the substrate doping is increased to too high a level.
In view of the above it has been proposed in GB-B-2113907 to provide a buried local highly doped n+ region at the first or main pn junction to control the breakdown voltage of the main pn junction. The local highly doped n+ region is provided within the n type central region immediately beneath the n type cathode region and spaced from the n type cathode region by the p type central region so that the n+ region is localized within the body of the device. As described in GB-B-2113907 the localized buried n+ region may be an ion-implanted phosphorus doped region which is overdoped by the central p region. The formation of such buried region can, however, be difficult to control, making it difficult to control with any precision the voltage at which the device will break down when the main pn junction is reverse-biassed. Furthermore, an additional masking stage is required to provide the buried region which necessarily increase the time and costs involved in production of the device. Additional background prior art is contained in EP-A-167440; U.S. Pat. Nos. 3,551,760 and 4,282,555; and GB-A-1300726.
According to one aspect of the invention, there is provided a semiconductor diode comprising a semiconductor body having a first region of one conductivity type, a second region of the opposite conductivity type meeting only a given surface of the body and surrounded by the first region so as to form with the first region a first pn junction which, when reverse-biassed in operation of the diode by a voltage applied across the diode, gives the diode a blocking characteristic, and a third region of the one conductivity type more highly doped than the first region provided within the first region for triggering conduction of the diode when a predetermined voltage less than the voltage at which the first pn junction would have broken down in the absence of the third region is applied across the diode to reverse bias the first pn junction, characterized in that the third region meets only the given surface and a passivating layer on the given surface covers the third region, the third region being located such that a depletion region extends from the first pn junction to the third region when a voltage less than the predetermined voltage is applied across the diode to reverse-bias the first pn junction and the predetermined voltage is determined by the relative locations of the second and third regions.
A further region meeting only the given surface and forming a pn junction with the first region may also be provided, the fourth region forming a breakdown device with the third region and being disposed between the second and the third regions.
In a particular embodiment of the present invention, a semiconductor diode in accordance with the invention may comprise a semiconductor body having a first region of one conductivity type, a second region of the opposite conductivity type meeting only a given surface of the body and surrounded by the first region so as to form with the first region a first pn junction which when reverse-biassed in operation of the diode by a voltage applied across the diode gives the diode a blocking characteristic and a third region of the one conductivity type more highly doped than the first region provided within the first region for triggering conduction of the diode when a predetermined voltage less than the voltage at which the first pn junction would have broken down in the absence of the third region is applied across the diode to reverse bias the first pn junction, characterized in that the third region meets only the given surface, a fourth region of the opposite conductivity type disposed between the second and third regions meets only the given surface and forms a pn junction with the third region and a passivating layer on the given surface covers the third and fourth regions, the third and fourth regions forming a breakdown device and the relative locations of the breakdown device and the second region being selected such that, when the first pn junction is reverse-biassed by a voltage applied across the diode, a depletion region of the first pn junction meets a depletion region of the breakdown device at an applied voltage less than the predetermined voltage and, when the predetermined voltage is applied across the diode, a reverse-biassing voltage induced across the pn junction between the third and fourth regions causes the breakdown device to breakdown to trigger conduction of the diode.
Thus, in a semiconductor diode embodying the invention the predetermined voltage at which the diode switches into conduction when the first pn junction is reverse-biassed can be controlled relatively easily by selecting the relative locations of the second and third regions. Moreover, third region meets the only the given surface so that a deep diffusion to provide a local highly doped region at the first pn junction is not necessary, thereby enabling more precise control of the voltage at which the diode will breakdown when the first pn junction is reverse-biassed compared to a diode in which a local highly doped region is provided at the first pn junction.
One or more additional regions of the opposite conductivity type may be disposed spaced-apart within the first region between the second and fourth regions to increase the reverse-biassing voltage at which the first pn junction would breakdown in the absence of the third region, the additional region meeting only the given surface and being covered by the passivating layer. The additional region(s) enable(s) greater control over the predetermined voltage to be obtained by selection of the number and position of the additional regions in addition to the selection of the location of the breakdown device. Thus by selecting the number of and position of the additional region any predetermined voltage within a large range of voltages may be selected, thus increasing the flexibility of the diode.
In an alternative arrangement, the third region may form a pn junction with and be surrounded by the second region so as to separate parts of the second region by a predetermined distance such that, when a voltage is applied across the diode to reverse bias the first pn junction, a depletion region of the first pn junction isolates the third region from the first region at an applied voltage less than the predetermined voltage and at the predetermined voltage the third region triggers conduction of the diode.
Normally, in such an alternative arrangement, the doping of the relatively highly doped third region relative to that of the first region will be such that when the predetermined voltage is applied across the diode to reverse bias the first pn junction, the reverse-biassing voltage induced across the pn junction between the second and third regions causes that pn junction to breakdown so triggering conduction of the diode.
In a further embodiment, the third region may be surrounded by the second region but separated from the second region by the first region, the predetermined voltage being determined by the separation of the third region by the first region from the second region. One or more additional regions of the opposite conductivity meeting only the given surface may be disposed spaced-apart between the second region and the third region and also between the fourth region and the third region to increase the reverse voltage at which the first pn junction would breakdown in the absence of the third region, the additional region(s) being covered by the passivating layer. Such additional region(s) enable increased control of the breakdown voltage by enabling selection of the number and position of the additional regions in addition to the position of the third region relative to the second region and also enable an increase in the flexibility of the diode by enabling any desired predetermined voltage within a large range of voltages to be selected by selecting the number and position of the additional regions.
The passivating layer may be an insulating layer or may be a semi-insulating layer. Where the layer is insulating, a resistive bleed layer may extend over the insulating layer from the first pn junction to the third region to isolate the semiconductor diode from its surroundings. Alternatively, a field plate may extend over the passivating layer from the first pn junction to the third region. Where additional regions are provided a respective field plate may be associated with each additional region.
The first and second region may form part of a gateless thyristor structure. The thyristor structure may, in addition to the first and second regions, comprise a fifth region of the one conductivity type more highly doped than the first region and disposed within the second region so as to meet only the given surface and a sixth region of the opposite conductivity type more highly doped than the second region disposed within the first region so as to meet only a further surface of the body opposed to the given surface. The thyristor structure may be arranged in antiparallel with an np or two region diode structure formed by the first and second regions, so providing an assymmetric breakover diode, or may be disposed in antiparallel with a second similar thyristor structure so forming a symmetric breakover diode. In the latter case, the semiconductor body may have a center of inversion symmetry such that the body appears identical when viewed from the given surface toward the further surface as when viewed from the further surface toward the given surface.
Where the semiconductor diode has the fifth region of the one conductivity type more highly doped than the first region mentioned above, for example where the diode has a gateless thyristor structure, in particular where the diode is a breakover diode, the third region may be formed at the same time as the fifth region and the fourth region (plus any additional region(s)) if present may be formed at the same time as the second region by using appropriate masks so that such a diode embodying the invention can be manufactured without any additional masking steps and without any significant increase in manufacturing times and/or costs.